Soon after birth, most infants develop the optimal refractive error (i.e., clinical emmetropia) in both eyes that is then maintained throughout childhood and into early adult life. However, for reasons not currently understood, a significant and increasing proportion of the population develop abnormal refractive errors. The long-term goal of our research program is to provide a better understanding of the etiology of common human refractive errors (e.g., myopia or nearsightedness) and ultimately to develop effective optical treatment strategies that reduce the impact of refractive errors. The specific aims of our proposed research are to determine how visual experience affects refractive development and to characterize the operational properties of the vision-dependent mechanisms that regulate eye growth. Since many of the required experiments can not be conducted in humans, but our purpose is to generate knowledge that can be applied to human development, these experiments will be conducted using rhesus monkeys. Controlled rearing strategies, optical and ultrasonographic measurement techniques, and magnetic resonance imaging will be used to determine: 1) the effects of high ambient light levels on emmetropization and vision-dependent changes in eye growth, 2) the influence of relative peripheral myopia on central axial growth and refractive development, and 3) whether relative myopic defocus that is simultaneously superimposed on competing hyperopic defocus (a very strong stimulus for growth) can retard axial growth and promote hyperopia. The proposed experiments focus on fundamental issues concerning the manner in which visual experience influences refractive development and are an important step in determining how and to what extent visual experience contributes to the genesis of common human refractive errors. More importantly, the results of these studies will potentially provide the foundation for new treatment and management strategies for human refractive errors.